Secondary dryer for veneer and like material

ABSTRACT

A secondary dryer for wood veneer and similar sheet material having widely varying initial moisture content for completing the drying of that portion of material (&#39;&#39;&#39;&#39;redry&#39;&#39;&#39;&#39;) which is underdried after initial passage through a primary dryer. The secondary dryer is arranged in a closed loop configuration, provided by an endless conveyor belt carrying the material in a vertically disposed elliptical path through an overhead drying chamber, so that underdried material is automatically recycled therethrough by the conveyor until the desired degree of final moisture content is achieved.

United States Patent [72] Inventors Henry W. Schuette;

Albert H. Barnes both 01 Portland. Oreg. 56299 July 20, 1970 Division 01' Ser. No, 810.996. Mar. 27. 1969, Pat. No. 3.545.094.

[2H AppLNo. [22] Filed (45] Patented July 13, 1971 [73] Assignee Moore Dry Kiln Company of Oregon North Portland, Oreg.

(54] SECONDARY DRYER FOR VENEER AND LIKE MATERIAL 4 Claims, 6 Drawing Figs.

[52] U.S. Cl t. 34/66, 34/67. 34/102, 34/162 [51] 1nt.C1 F26b 19/00 [50] Field otSearch 34/66,67, 102, 149, 162

[56] References Cited UNlTED STATES PATENTS l.436,037 11/1922 Heliot o. 34/67 3,199,213 8/1965 Milligan et a1. 34/13.8

Primary Examiner-Carroll B. Dority, .lr. AttorneyDaniel P. Chernoff ABSTRACT: A secondary dryer for wood veneer and similar sheet material having widely varying initial moisture content for completing the drying of that portion of material (redry") which is underdried after initial passage through a primary dryer. The secondary dryer is arranged in a closed loop configuration, provided by an endless conveyor belt carrying the material in a vertically disposed elliptical path through an overhead drying chamber, so that underdried material is auto matically recycled therethrough by the conveyor until the desired degree of final moisture content is achieved.

SECONDARY DRYER FOR VENEER AND LIKE MATERIAL CROSS-REFERENCE TO RELATED APPLICATION The present application is a division of a pending application of the same inventors, Ser. No. 810,996 filed Mar. 27, 1969, now U.S. Pat. No. 3,545,094 granted Dec. 8, I970 BACKGROUND OF THE INVENTION This invention relates to apparatus for the production drying of wood veneer and similar sheet material whose initial moisture content varies over a wide range. More particularly, the present invention is directed to a secondary dryer which receives that portion of material emerging from a primary dryer chamber which, because of its high initial moisture content, is underdried and subjects this redry material to a further drying until the desired moisture level is reached. A closed loop conveyor design for the secondary dryer permits the underdried material to be automatically recycled through the secondary dryer chamber as many times as necessary in order to achieve the requisite degree of dryness in the final product. Whereas the exemplary embodiment of the invention disclosed herein relates to the drying of wood veneer, it is to be understood that these teachings are also advantageously applicable to the production-line drying or heat treatment of other sheet or ribbonlike materials whose initial moisture content varies over a wide range. I

For many years the conventional method of production drying wood veneer has been to handle the product in sheet form and pass it in single layers through a multideck conveyor-type drying chamber. The veneer is usually peeled from the log by a rotary lathe in a variable sheet thickness dependent upon the size desired of the final multi-ply panel product. The outer layers of the log when peeled by the lathe are generally considered sapwood and as a rule possess the higher moisture content, whereas the inner layers, referred to as heartwood, have lower moisture content and in general possess a somewhat different texture and density. While in some species of timber the differential in moisture content between heartwood and sapwood is not as great as in others, nevertheless, a measurable differential does exist in virtually all cases.

In those wood species having a great range of moisture content differential between heartwood and sapwood, an attempt is usually made to segregate the two immediately after the green veneer is clipped into sheet sizes. Typically this segregation is accomplished, where possible, by utilizing the natural color differential which exists between heart and sup material, but more often by the trained eye of the sorter recognizing certain distinctive characteristics of the inner wood in contrast to the outer layers. Although this segregation of heart from sapwood has heretofore been carried out manually, recently moisture detectors have been developed to accomplish this segregation automatically; however, at the present state of the art, such instrumentation has not yet received wide-scale acceptance by the industry and lacks the broad band resolution capability for handling material having a wide range of moisture content.

Although some veneer drying is now being carried out in the industry with the wood material in ribbon form or as sheets on a batch-type tray, in the great bulk of plywood production the veneer is in sheet form and is processed for drying in packages in which single-layered sheets are continuously fed into multideck conveyor dryers. After an initial pass through the drying chamber, the sheet veneer is graded and again stacked into dry packages. Even where there has been an initial segregation of heartwood from sapwood material, typically a wide differential in moisture content still exists among the veneer sheets after this first pass through the dryer. In fact, in order to dry the majority of a typical run to a satisfactory moisture level, as much as 70 percent of the run might be overdried to a less than desirable moisture level. In recognition of the wide moisture differential which can exist in the veneer package after the initial drying run, the industry in general follows the practice of setting aside a s mall percentage of each run as redry for further processing. In conventional practice this redry material (that is, material which has been insufficiently dried in the initial drying run because of a higher-than-average initial moisture content) is segregated from the acceptably dried material and set aside until enough has been accumulated to warrant a second run of this material through the drying chamber. However, because the handling of this redry material during the second drying run results in considerable degrade and breakage, sizable production loss and decreased drying efficiency, the industry practice is to allow but a small percentage of material as redry. Perhaps the main objection to processing redry veneer, so as to reduce its moisture content to an acceptable level, is that it ties up the dryer and thus interrupts the mainline processing of green veneer in the production run. Also, the dryer itself is caused to be operated at low efficiency during a redry run because substantially reduced temperatures are required in the circulating drying medium in order to prevent overdry of this partially dried material.

SUMMARY OF THE INVENTION The present invention is directed to a secondary dryer for use exemplarily in conjunction with a primary dryer in a multi ple-dryer system which overcomes the need for initial segregation of the veneer product while in the green state to separate the high-moisture level sapwood from the less wet heartwood material. The two-dryer system arrangement provides a means for efficiently drying the underdried material which results from the initial drying process without additional handling and without interrupting the production run of green veneer through the drying system.

The first dryer, which is of conventional multideck conveyor-type design, acts as a primary dryer through which is processed all of the green veneer material which may be either in sheet or ribbon form. The operating parameters (i.e., conveyor speed, drying air temperature and circulation flow) of this primary dryer are adjusted so that only the veneer material of initially low moisture contents is fully dried during passage therethrough. A conventional moisture detector is located at the outfeed or dry end of the primary dryer to differentiate the fully dried material, typically 30 to 50 percent of the throughput of the primary machine depending upon species, from the remainder requiring further drying. The material which is satisfactorily dried during passage through the primary dryer, as determined by the moisture detector, is pulled off the conveyor, graded, put up into packages, and is then ready for layup into plywood panels. The remaining material, which is marked by the moisture detector as insufficiently dried, continues on the conveyor to a secondary dryer of relatively lower production capacity in contrast to the primary dryer. In the transition of the material by the conveyor from the primary to the secondary dryer, the'underdried veneer material is not handled or packaged in any way, and therefore the likelihood of breakage or degrade is very substantially reduced, if not entirely eliminated.

In the exemplary embodiment of the two-dryer system designed to handle veneer sheets in contrast to veneer ribbon, the conveyor arrangement between the primary dryer outfeed and the infeed to the secondary dryer is so constructed that the grain orientation in the sheets is rotated with respect to the flow direction so that, for example, if the material is initially processed through the primary dryer with its grain orientation parallel to the flow direction, it will be received in the secondary dryer with its grain pattern perpendicular to the flow in the secondary unit. 0n the other hand, where the material being handled is in ribbon veneer form, the primary and secondary dryers would accordingly be arranged in-line and there would be no alteration of grain direction in the transition of the material from the primary to the secondary dryer.

The secondary dryer of the present invention, exemplarily designed for operation in a series dryer arrangement, is constructed to form a closed loop system for material entering the secondary dryer. The closed loop design of the secondary dryer, provided by an endless conveyor belt which carries the veneer material in a vertically disposed elliptical path through an overhead drying chamber, permits the veneer to be automatically recycled through the secondary dryer as many times as necessary in order to reduce its moisture content to a satisfactory level. Recycling of some of the material through the secondary dryer is necessary because, in spite of the segregation which occurs at the dry end of the primary dryer, the material which reaches the second dryer still possesses a high moisture content which varies over a wide range. In order to monitor the moisture content of the material after its passage through the secondary dryer, a second moisture detector is located at the outfeed end of the recirculating conveyor of the secondary system to mark material whose moisture content still remains above the desired level. In a typical wood species some 70 percent of the veneer material entering the secondary dryer would be satisfactorily dried after a single pass, but the remaining 30 percent would require at least a second pass through the secondary drying chamber.

As previously indicated, one of the principal significant advantages of the primary-secondary dryer system summarized above is that the veneer product is not damaged or degraded by handling during the drying process. Further, none of the veneer material is deleteriously overdried, as is typical in conventional systems designed to minimize the amount ofredry" material, and thus the undesirable effects in plywood panel production of excess glue usage and delamination due to glueline dryout and poor veneer surface condition are avoided. Finally, a most significant advantage of the present system is that the efficiency of the drying operation is greatly increased, as the material passing through the system is dried only to the desired moisture level and not substantially below that level. Also, the period of time during which the veneer material is exposed to the drying air medium is reduced, thus permitting higher production speeds, or conversely, the operative area of the drying chamber can be reduced as much as 20 to 30 percent while handling the same volume of veneer material as that dried by prior art unitary dryer systems.

It is therefore a principal objective of the present invention to provide a novel apparatus for effecting secondary production drying wood veneer and similar sheet material having widely varying initial moisture content.

It is a principal feature of the present invention that veneer material, which being of initially high is an enlarged view showing a detail of the content is underdried after passage through a primary drying chamber, is processed in a secondary drying chamber, and that any such material which remains underdried after passage through the secondary dryer is automatically recycled through the secondary dryer as many times as necessary to achieve the desired moisture level.

The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS u in FIG. 2.

FIG. 4A is an enlarged view showing a detail of the drying chamber depicted in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, in FIG. I there is shown an exemplary veneer dryer system arrangement. The primary dryer, indicated schematically at 110, may be one of the conventional types now used in the industry for the drying of wood veneer such as, for example, the dryer manufactured and sold by applicants assignee under the trademark UNI- .IET which comprises a multideck gas or steam heated air dryer with a plurality of drying sections.

As previously indicated, the operating parameters of the primary dryer 10 are adjusted so that, of a typical unsegregated heartwood and sapwood run of green veneer supplied to its infeed ll, between 30 to 50 percent are satisfactorily dried, with no overdry, after passage through the primary drying chamber. In other words, only that portion of the run which is of initially low moisture content is completely dried to the requisite moisture level by processing through the pri mary dryer [0. After emergence from the primary dryer 10, the veneer material received at the primary dryer outfeed 12 is individually scanned by a conventional moisture detector 15 which discriminates between those veneer pieces which have been dried to a moisture content at or below a predetermined level from those which require further drying. In a typical moisture detector device, the partially dried material is sprayed with a water soluble colored dye so that it can be readily distinguished. One suitable type of moisture detector unit for use in this application is the one manufactured and sold by applicant's assignee as the Moore Oregon Continuous Check Moisture Detector described in its bulletin No. 6608.

After passage past the moisture detector 15, the veneer material, which in this embodiment is handled in the form of sheets, is transferred to a conveyor 20 where its orientation is altered relative to the initial direction of travel. This changes the orientation of the veneer grain relative to the path of travel of the material through the secondary dryer 30. Thus, typically, if the grain orientation of the veneer through the primary dryer I0 were parallel to the direction of travel, the right angle turn indicated at 20a would change the grain orientation so that in passage through the secondary dryer 30 the wood grain would be perpendicular to the direction of travel.

Before reaching the secondary dryer 30 the veneer passes by a grading station, indicated as 25ah, where graders pull off the unmarked, fully dried veneer and sort it according to conventional grading techniques. The remainder of the veneer material, marked as being insufficiently dried, is carried by the conveyor 20 into the infeed 31 of the secondary dryer 30. As best shown in FIGS. 2 and 3, the thin, partially dried veneer sheets 18 carried to the infeed 31 of the secondary dryer 30 are transported by a conveyor web 35 in a closed elliptical path, first upwardly and then through an overhead-mounted heat treatment chamber 36, then downwardly and past a second moisture detector 37 of conventional design, next past a grading station 39, and finally return to the beginning of the loop.

Thus, in the aforedescribed system arrangement, all partially dried veneer material received from the primary dryer I0 is fed through the drying chamber of the secondary dryer 30 where its moisture content is further reduced. If the material is now satisfactorily dried after an initial pass through the secondary drying chamber 35, it is pulled off the conveyor web 35 by graders at the grading station 39 onto trolleys 390 --f (FIG. I) from whence it may be taken to the plywood panel layup area. On the other hand, veneer material which is still too wet for use is marked by the second moisture detector 37, and is then carried by the conveyor past the grading station and on to a recycling through the secondary drying chamber 36. In this manner all of the veneer material, regardless of its initial state of wetness, can be fully dried to the required moisture level without handling or manual intervention at any intermediate stage of the drying operation.

Specific structural details of the exemplary secondary dryer embodiment are depicted in FIGS. 2--4A. As shown in the figures, the heat-treating chamber 36 is supported overhead (i.e., some 8 to 10 feet above the ground) on a platform 41 by a structural skeleton of supporting shores 42 and cross braces 43.

Carried on the platform 41 is the heat-treating chamber comprising a first section 45 in which a heated air medium is directed at high velocity against the surfaces of the veneer material to evaporate moisture therefrom, and a second section 55 in which ambient air is circulated to cool the veneer and also carry away remaining moisture. As best shown in FIGS. 4 and 4A, a pair of centrifugal fans 46 in the heating section 45 pressurizes air which is also heated to high temperature (typically, above 350 F.) by a gas burner (not shown). The pressurized heated air is then forced into a respective first plenum chamber 47 and thence into an associated hollow air box 48 disposed laterally on either side of the veneer material which is transported through the central core area. As indicated in FIG. 4A, the veneer 18 is carried through the center of the chamber by means of a sandwiching conveyor web 35a and 35b supported and driven by a roller mechanism 49.

The heated air emerges from the respective air boxes 48 at high velocity, typically 2,500 l0,000 linear feet per minute, through a plurality of jet nozzles 52 spaced in a staggered arrangement to direct the heated air medium perpendicularly to the horizontal surfaces of the moving veneer material 18. The high-velocity, high-temperature jet action produces an efficient and rapid drying of the partially dried veneer material and, because of the disruption of the interface boundary layer which occurs due to the sharp right-angle impingement of the jet streams on the flat surface of the sheet material, the drying action is extremely thorough. The high air temperatures used, which are considerably above those conventionally employed in veneer dryers, are not injurious because the moisture is rapidly drawn from the interior of the veneer material and quickly evaporated from the surface, thereby producing a cooling efiect which protects the board from injury or discoloration. A more detailed explanation of the drying phenomena associated with this type of high-temperature, high-velocity impingement jet action is set forth in U.S. Pat. No. 3,199,213 whose disclosure is herein incorporated by reference.

After impingement on the veneer material, the air flow is swept laterally across the surface thereof and exhausted from the central core area and into the respective second plenum passages 51, and thereafter drawn toward the inlet side of the opposing centrifugal fan 46 to complete the circulation path. The moisture carried away from the veneer material in the drying process and collected in the gaseous medium is then exhausted to the atmosphere through the vent stack 53.

After passage through the heating section 45, the veneer material then passes into the adjacent cooling section 55 of similar design in which unheated ambient air is directed in high velocity jet streams perpendicularly against the respective faces of the veneer sheet so as to draw off remaining moisture. In this cooling section the air is circulated in a fashion similar to that in the heating section by means of a single centrifugal fan 56, and the moisture-laden air is exhausted to the atmosphere by a separate stack 58 provided in the cooling section.

The conveyor mechanism 35 which transports the veneer material 18 in a closed loop configuration through the secondary dryer 30 is of generally conventional design utilizing powered and idler rollers driven by suitable mechanisms known to the art. In order to secure the veneer material 118 during its passage through the heat-treating chamber where high-velocity jet air flows are encountered, the veneer is preferably constrained in sandwichlike fashion by the conveyor web 35 which, as shown in H0. 4A, overlies the top and bottom of the material during this portion of its travel. ln its arcuate vertical passage from the overhead-mounted drying chamber to the ground level moisture detection and grading stations, and then back again, the veneer material is supported, as shown in FIG. 3A, by an overlying conveyor belt portion 35c which presses the veneer material 18 against a plurality of closely spaced idler rollers 35 disposed along the arcuate path. It will be noted from the nature of the conveyor mechanism 35 shown in FIG. 3 that the veneer sheets 18 will continue to recirculate in the closed path between the drying chamber and the grading station until the steel material is physically taken off the conveyor belt. 7

While the described embodiment of the present invention has been directed to a system for handling veneer in sheet form, it will be apparent that the system is readily adaptable for the handling of veneer in ribbon form by the alignment of the primary and secondary dryers in a colinear arrangement and, in such modification, some lengthening of the elliptical path traveled by the conveyor belt of the secondary dryer system might be required in order to deal with the longer ribbon lengths involved.

lLLUSTRATlVE EXAMPLE In an exemplary operation of the primary-secondary dryer system disclosed herein, samples of intermixed sapwood and heartwood green veneer, having a range of initial moisture content varying from 35 percent to 158 percent (based, as is the conventional industry practice, on the bone dry weight of the veneer) were entered into the infeed of the primary dryer unit. The operating parameters of the primary dryer were set so that only the material having the lowest initial moisture content was fully dried in this unit; thus, the material of originally 35 percent moisture level was reduced to a satisfactory moisture level of 4 percent in its passage through the primary unit, and this material was then removed from the system. However, the veneer material of initially 158 percent moisture content was reduced to only a partially dry 22 percent moisture level after passage through the primary dryer, and this wet material was thereafter continued on to the secondary dryer where it was finally reduced to a satisfactory 3% percent moisture level in two passes through the secondary dryer unit. Material of moisture content intermediate these two extremes required but a single pass through the secondary dryer to reach the desired moisture level. In this manner all of the material was dried to a satisfactory moisture level within a narrow range centering around 2 4 percent moisture content, with none of the material being underdried and none of it requiring further handling or reprocessing as redry material.

What we claim is:

1. Apparatus, for continuous heat treating of planar material, of the type comprising a drying chamber having a horizontal core opening extending longitudinally through the center thereof, a conveyor for carrying said material through said chamber core, and a plurality of nozzles arranged in a spaced array throughout said chamber and coupled to a source of pressurized heated air for directing jets of high temperature air at high velocity and substantially perpendicularly to the major surfaces of said material advancing through said chamber core, characterized in that said chamber is supported overhead and above ground level by a skeletal framework so as to form an exposed open area underneath said chamber and that said conveyor is arranged in a vertically disposed closed loop so that said material, after passage through said chamber core, is transported downward, then through said open area directly underneath said chamber, and finally upward to return to the input of said chamber, whereby material will be repetitively cycled through said drying chamber until removed from said conveyor.

2. The apparatus of claim 1 further characterized in that a moisture detector means for discriminating fully dried from partially dried material is positioned to examine material on said conveyor after its passage through said chamber core, and a grading station is provided underneath said chamber in said open area at which personnel may remove fully dried material from said conveyor as marked by said moisture detector.

3. An apparatus according to claim 1 further characterized in that said drying chamber comprises two zones, a first zone in which high temperature heated air is directed in a plurality of jet streams against the major surfaces of said material passing therethrough, and a second zone in which unheated air is directed in a plurality of high velocity jet streams against said surfaces to cool said material passing therethrough.

4. The apparatus of claim ll characterized in that said conveyor for transporting said planar material in a closed loop path comprises:

a. during travel of said material through said drying UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,591,927 Dated uly 13, 1971 Inventor) Henry W. Schuette and Albert H. Barnes It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, Line 37 Change multideck" to multi-deck-.

Col. 3, Line 46 After "drying" insert of;

Lines 49-50 Change "is an enlarged view showing a detail of the" to moisture;

Lines 68-69 Delete "is an enlarged view showing a detail of;

Line 71 After "is" insert an enlarged view showing a detail of the-.

Col. 6, Line 6 Change "steel" to sheet-.

Signed and sealed this 29th day of February 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. Apparatus, for continuous heat treating of planar material, of the type comprising a drying chamber having a horizontal core opening extending longitudinally through the center thereof, a conveyor for carrying said material through said chamber core, and a plurality of nozzles arranged in a spaced array throughout said chamber and coupled to a source of pressurized heated air for directing jets of high temperature air at high velocity and substantially perpendicularly to the major surfaces of said material advancing through said chamber core, characterized in that said chamber is supported overhead and above ground level by a skeletal framework so as to form an exposed open area underneath said chamber and that said conveyor is arranged in a vertically disposed closed loop so that said material, after passage through said chamber core, is transported downward, then through said open area directly underneath said chamber, and finally upward to return to the input of said chamber, whereby material will be repetitively cycled through said drying chamber until removed from said conveyor.
 2. The apparatus of claim 1 further characterized in that a moisture detector means for discriminating fully dried from partially dried material is positioned to examine material on said conveyor after its passage through said chamber core, and a grading station is provided underneath said chamber in said open area at which personnel may remove fully dried material from said conveyor as marked by said moisture detector.
 3. An apparatus according to claim 1 further characterized in that said drying chamber comprises two zones, a first zone in which high temperature heated air is directed in a plurality of jet streams against the major surfaces of said material passing therethrough, and a second zone in which unheated air is directed in a plurality of high velocity jet streams against said surfaces to cool said material passing therethrough.
 4. The apparatus of claim 1 characterized in that said conveyor for transporting said planar material in a closed loop path comprises: a. during travel of said material through said drying chamber, a pair of longitudinally extending paralleling belts of open-weave mesh construction which constrain said material therebetween in sandwichlike fashion, and b. during vertical passage of said material between said overhead-mounted drying chamber and ground level, a longitudinally extending belt portion which overlies said material and presses it against a plurality of closely spaced idler rollers disposed in an arcuate vertical path between said drying chamber and said ground level return path. 